The core plate assembly separates the core of fuel assemblies of the boiling water reactor from the lower plenum provided beneath the core plate assembly.
U.S. Pat. No. 5,646,970 discloses a control rod guide tube and a core plate with a plurality of through-going apertures. A transition piece for four fuel assemblies is provided in each of the through-going apertures and comprises four passages for coolant to the respective fuel assembly. Flow inlets to the passages are provided. The flow inlets are formed by a respective orifice which has a circular shape.
U.S. Pat. No. 4,904,443 discloses a control rod drive with upward removable drive internals, and a core plate with a plurality of through-going apertures. A transition piece for four fuel assemblies is provided in each of the through-going apertures and comprises four passages for coolant to the respective fuel assembly. Flow inlets to the passages are provided. The flow inlets are formed by a respective orifice which has a circular shape.
When the coolant is flowing from the lower plenum through the flow inlet into the passage of the transition piece, vortices are created due to the complex geometry outside the transition piece. These vortices fluctuate at the orifice of the flow inlet, which can lead to a pulsating mass flow through the orifice. Thus, the pressure in and behind the orifice will fluctuate, which leads to a pressure loss coefficient through the orifice that varies with time. This will cause an intermittent mass flow through the respective passage and fuel assembly that can lead to a cooling deficiency, and in a worse case can cause dry-out events in the fuel assembly.